That means outreach targeted to select areas can pay off big, reaching millions of prospective customers needed to stabilize the law's new insurance markets.

The pattern also holds true for the younger uninsured, the health care overhaul's most coveted demographic. The study found that half of uninsured people ages 19-39 live in 108 counties. Their premiums are needed to offset the cost of care for older adults.

With most of the bugs out of the HealthCare.gov website, the Obama administration is using the geography of the uninsured to write a playbook for its closing sign-up campaign.

Enrollment ends March 31 for subsidized private insurance, available to people who don't have coverage at work. But many who could benefit are procrastinating. Some people are confused by the new law. Others don't think they will qualify for help.

"Our efforts are aimed at making sure we can raise awareness in areas with the largest concentration of uninsured people," said Julie Bataille, communications director for the rollout at the federal Health and Human Services Department.

The administration has done its own geographical research, drilling down even below the county level.

With their own research, federal officials are focusing on 25 key metro areas. The top two are in Texas: Dallas and Houston. Next come Miami and Atlanta. In the Northeast, the northern New Jersey megalopolis and Philadelphia are on the list. Midwest markets include Detroit, Cleveland and Indianapolis. Southern cities also include Nashville and Charlotte, N.C.

Washington is largely steering clear of states that are leading their own sign-up efforts, such as California, New York and Illinois.